1. Field of Invention
The present invention relates to a magnetic recording medium and a magnetic recording apparatus. In particular, the present invention relates to a magnetic recording medium which is excellent in thermal stability and which is preferably usable for high density recording. The present invention also relates to a magnetic recording apparatus installed with the same.
2. Description of the Related Art
The multimedia, which makes it possible to process not only character information but also voice and image information at a high speed, comes into widespread use in accordance with the development of the advanced information society in recent years. A magnetic recording apparatus, which is installed to a computer or the like, is known as one of those related to the multimedia. At present, the development is advanced in order to realize the miniaturization while improving the recording density of the magnetic recording medium to be used for the magnetic recording apparatus.
A typical magnetic recording apparatus comprises a plurality of magnetic disks which are rotatably installed to a spindle. Each of the magnetic disks is composed of a substrate and a magnetic film formed thereon. Information is recorded by forming magnetic domains having specified magnetization directions in the magnetic film.
The in-plane magnetic recording system is adopted for the magnetic recording apparatus practically used at present. The in-plane magnetic recording system is based on the use of a magnetic recording medium which comprises, as a recording layer, a magnetic layer having an easy axis of magnetization in a direction parallel to the disk surface so that magnetic domains having in-plane magnetization are formed in the recording layer to perform the recording. In contrast to the recording system as described above, the perpendicular magnetic recording system has been suggested as a recording system to realize high density recording on a magnetic recording medium. The perpendicular magnetic recording system is based on the use of the magnetic recording medium which has perpendicular magnetic anisotropy so that magnetic domains having perpendicular magnetization are formed in a recording layer to perform the magnetic recording.
In any one of the recording systems, in general, a Co—Cr-based alloy is used for the recording layer of the magnetic recording medium. Owing to the effect of the added element, the Co—Cr-based alloy undergoes the phase separation into two types of phases, i.e., a ferromagnetic phase in which the Cr concentration is low and a non-magnetic phase in which the Cr concentration is high. Therefore, the Co—Cr-based alloy makes it possible to reduce the magnetic interaction between crystal grains (N. Inaba et al., J. Appl. Phys. 87, 6863 (2000)). That is, in the Co—Cr-based alloy, the magnetic coupling, which would be effected between the crystal grains, is broken by the non-magnetic layer in which the Cr concentration is high. Therefore, the crystal grains are magnetically isolated from each other in the Co—Cr-based alloy. Accordingly, any zigzag domain wall, which would cause the medium noise, is not formed at the bit boundary, i.e., at the portion of the magnetization transition area. Thus, the reduction of the medium noise is realized.
Although the Co—Cr-based alloy has the advantage as described above, it involves such a problem that the crystalline magnetic anisotropy is small, i.e., about 2×106 erg/cm3. For example, the Co—Cr-based alloy has had the following problem. That is, when the magnetic grains of the Co—Cr-based alloy for forming the recording layer are made fine and minute in order to realize a higher recording density with the magnetic recording apparatus, the thermal stability of the magnetic grains is degraded, because the magnetization reversal volume is decreased. In order to solve the problem of the thermal stability as described above, a method has been suggested, in which a material having crystalline magnetic anisotropy larger than that of the Co—Cr-based alloy is used for the material for forming the recording layer. Those known as the material having the large crystalline magnetic anisotropy include L10 ordered alloys such as FePt, CoPt, and FePd. The crystalline magnetic anisotropy constant of the L10 ordered alloy as described above is 1.8 to 10×107 erg/cm3 which is larger than that of the Co—Cr-based alloy by not less than one digit. Therefore, it is possible to expect high thermal stability even when the magnetic grains are made fine and minute.
However, when the L10 ordered alloy is simply used to form a recording layer, it has been unsuccessful to obtain any two-phase separation structure composed of the magnetic phase and the non-magnetic phase, unlike the Co—Cr-based alloy. Therefore, the following problem has arisen. That is, the magnetic interaction between the crystal grains is not broken any longer. A plurality of crystal grains undergo the magnetization reversal in bulk when a recording magnetic field is applied with a magnetic head. As a result, the noise is increased when the reproduction is performed. Therefore, when the L10 ordered alloy is used as a recording layer, it has been demanded to form a structure in which the magnetic interaction between crystal grains is reduced in the same manner as in the Co—Cr-based alloy.
In order to respond to this demand, for example, an attempt has been made to use a method in which a structure similar to that of the Co—Cr-based alloy is formed by dispersing an L10 ordered alloy in a base phase of oxide. However, this method has involved such a problem that it is difficult to control the orientation of the recording layer. Further, the ordering temperature of the L10 ordered alloy, which is required when the L10 ordered alloy is dispersed in the base phase of oxide, is higher than that required when only the L10 ordered alloy is manufactured. Therefore, there has been such a problem that any glass substrate, which cannot endure the high temperature treatment, cannot be used as a substrate for the magnetic recording medium (C. Chen et al., J. Appl. Phys. 87, 6947-6949 (2000)).